Phospholipid fatty acid tails spontaneously cluster together away from the aqueous cellular interior and exterior environment due to their nature.

The answer is hydrophobic.

Phospholipid fatty acid tails are made of fatty acids that are hydrophobic in nature. These tails face each other in the phospholipid bilayer making the inner part of the membrane hydrophobic. The heads of phospholipids are made of phosphates therefore they are hydrophilic. These molecules are called amphipathic molecules.